Frequency conversion of an intercepted radio frequency signal by mixing with one or more local oscillator signals is a well known technique used in radio receivers to convert the carrier frequency of the intercepted radio frequency signal to a lower, intermediate frequency (IF) signal. The IF signal is then demodulated to recover information which has been included in the intercepted signal. In one class of receivers, sometimes referred to as "zero IF" receivers, the intercepted signal is mixed with the one or more local oscillator signals to produce an IF signal having no carrier frequency. In such zero IF receivers, dual conversion is often used, wherein a first local oscillator signal is mixed with the intercepted signal, thereby producing a first IF signal, which is then mixed with a second local oscillator signal to produce a second IF signal having no carrier frequency. Dual conversion receivers are also used in receivers which are not zero IF receivers, that is, the demodulation is performed using a second IF signal having a carrier frequency.
Known means of performing the mixing in dual conversion receivers are to use a first transistor circuit commonly identified as a Gilbert cell to generate the first IF signal, which is fed into the inputs of a second Gilbert cell transistor circuit to produce the second IF. While series connected mixer circuit approaches such as the Gilbert cell approach described above have worked well, each mixer circuit is typically independently powered from a common power supply, and the mixer circuits must be carefully matched to each other to reduce the generation of undesirable distortions of the signal, such as intermodulation distortion. In radio receivers which are portable and competitively priced, such as pagers, minimal circuitry, low power consumption, receiver sensitivity performance, and receiver intermodulation performance are extremely important characteristics.
Thus, what is needed is a mixer circuit for performing dual conversion which has lower power consumption, fewer parts, and less distortion.